Distance determination based on reflected admittance

Abstract

A distance measurement system includes a tank circuit including a transmit coil coupled to a transmit capacitor, a distance calculation circuit coupled to the transmit coil, and a target resonant circuit including a receive coil coupled to a receive capacitor. The receive coil is to receive a magnetic field generated by the tank circuit. The distance calculation circuit is to determine a reflected admittance. The reflected admittance includes a real component and an imaginary component. The distance calculation circuit is to determine a distance between the transmit and receive coils based on the imaginary component of the reflected admittance.

What is claimed is: 
     
       1. A distance measurement system, comprising:
 a tank circuit including a transmit coil coupled to a transmit capacitor; 
 a distance calculation circuit coupled to the transmit coil; and 
 a target resonant circuit including a receive coil coupled to a receive capacitor, wherein the receive coil is to receive a magnetic field generated by the tank circuit; 
 wherein the distance calculation circuit is to determine a reflected admittance, wherein the reflected admittance includes a real component and an imaginary component, and wherein the distance calculation circuit is to determine a distance between the transmit and receive coils based on the imaginary component of the reflected admittance. 
 
     
     
       2. The distance measurement system of  claim 1  further comprising a current sensor to sense current through the transmit coil and to provide the sensed transmit coil current to the distance calculation circuit, and wherein the distance measurement circuit is to:
 receive a transmit coil voltage; and 
 perform intermediate frequency sampling of the transmit coil voltage and sensed transmit coil current to produce real and imaginary components of the transmit coil voltage and real and imaginary components of the transmit coil current. 
 
     
     
       3. The distance measurement system of  claim 2  wherein the distance measurement circuit is to determine real and imaginary components of a back electromotive force (BEMF) based on the real and imaginary components of the transmit coil voltage. 
     
     
       4. The distance measurement system of  claim 3  wherein the distance measurement circuit is to determine the distance based on the real and imaginary components of the BEMF and based on the imaginary component of the transmit coil current. 
     
     
       5. The distance measurement system of  claim 3  wherein the distance measurement circuit is to determine the distance based on the ratio of the imaginary component of the transmit coil current to the sum of the real and imaginary components of the BEMF. 
     
     
       6. The distance measurement system of  claim 5  wherein the distance measurement circuit is to use the ratio to retrieve the distance from a look-up table. 
     
     
       7. The distance measurement system of  claim 1  wherein the distance calculation circuit is to determine a temperature value based on the real component of the admittance. 
     
     
       8. The distance measurement system of  claim 1  wherein the distance calculation circuit is to:
 determine a coefficient of coupling between the transmit and receive coils using the imaginary component of the reflected admittance; 
 determine the resistance of the receive coil using the coefficient of coupling; and 
 determine a temperature value based on the determined resistance of the receiver coil. 
 
     
     
       9. The apparatus of  claim 1  further comprising a current sensor to sense current through the transmit coil and to provide the sensed transmit coil current to the LDC, and wherein the LDC is to:
 receive a transmit coil voltage; 
 perform intermediate frequency sampling of the transmit coil voltage and sensed transmit coil current to produce real and imaginary components of the transmit coil voltage and real and imaginary components of the transmit coil current; and 
 determine real and imaginary components of a back electromotive force (BEMF) abased on the real and imaginary components of the transmit coil voltage. 
 
     
     
       10. The apparatus of  claim 9  wherein the LDC is to determine the distance based on the real and imaginary components of the BEMF and based on the imaginary component of the transmit coil current. 
     
     
       11. The apparatus of  claim 9  wherein the LDC is to determine the distance based on the ratio of the imaginary component of the transmit coil current to the sum of the real and imaginary components of the BEMF. 
     
     
       12. The apparatus of  claim 11  wherein the LDC is to use the ratio to retrieve the distance from a look-up table stored on the distance measurement circuit.